1. Field of the Invention
The present invention relates to internal combustion engines and, more particularly, to a two-stroke, homogeneous charge spark ignition (HCSI) engine cycle designed to solve the major obstacles preventing the commercialization of homogeneous charge compression ignition (HCCI) engines.
2. Background
Over the past several years, homogeneous charge compression ignition (HCCI) engines have held the promise of providing cleaner burning and more fuel efficient internal combustion engines. Characterized by the autoignition of a compressed lean homogenous charge, the entire compressed fuel/air mixture burns simultaneously avoiding further compression of already burned gases, which is the primary cause for the high combustion temperatures that cause the formation of NOx. Several obstacles, however, have thus far hindered the development of a commercially viable HCCI engine. Over-expanded HCCI engines are described in U.S. Pat. No. 7,114,485 to Pien, the specification of which is incorporated herein by reference.
Current HCCI engine research has focused on the four-stroke engine. For a four-stroke engine, the expansion ratio and geometric compression ratio are the same and equal to the ratio between cylinder total volume and cylinder clearance volume. The effective compression ratio, however, is the ratio between the air density within the cylinder clearance volume and the density of the ambient air. Since the air density in the clearance volume is controlled by the throttle valve or a supercharger, the effective compression ratio of a four-stroke engine is a variable, while the expansion ratio is fixed.
In HCCI engines, it is difficult to control autoignition and to operate at the required range of operating loads because of the difficulty of controlling the chemical kinetics of combustion over a range of loads. Moreover, with a four-stroke engine configuration, achieving high fuel efficiency requires a high compression ratio, which leads to high combustion temperature and NOx formation. The two-stroke HCSI engine employs a spark to trigger the flashpoint of a homogenous charge to achieve HCCI-like combustion.
With HCCI combustion, the whole fuel/air mixture burns at the same time and no part of the products of combustion is compressed into a higher temperature. Autoignition will take place whenever the fuel/air mixture is compressed to reach a flashpoint. As long as combustion temperature is less than the threshold temperature of NOx formation, lean HCCI combustion is emission free. When a lean homogeneous charge is compressed to a temperature close to, but below, the flashpoint, combustion of the charge initiated by a spark is close to emission free. At high loads where the threshold temperature for NOx formation may be exceeded, combustion will be emission-free except for NOx.
To prevent knocking and engine damage at high-loads, the compression ratio of an HCSI engine must be greatly reduced. Such reduction of the compression ratio, however, will not diminish engine thermal efficiency since engine thermal efficiency is already determined by the fixed expansion ratio.